How can sediments help scientists understand the motion of the ocean floor?
Just as seawater constantly changes with the flow of currents, so does the seafloor. Most of the bottom receives a constant rain of sediment. Sediment consists of organic and inorganic particles accumulated in a loose, unconsolidated form. As you'll see, sediment has many origins. Erosion, deceased hard-shelled organisms, chemical processes, volcanic activity, and outer space all contribute to the formation of ocean floor sediments. Scientists use these sediments to understand Earth's past climate, ocean circulation patterns, and biological developments, among other things. Sediments have given us clues to solving some big mysteries. For example, scientists have used information from sediments to develop hypotheses on the extinction of the dinosaurs, and about how dust from Africa may be damaging corals in the Caribbean.
Plate motion at a mid-ocean ridge The study of sediments can yield important information about the movements of the ocean floor. At the crest of the mid-ocean ridge, there is little or no sediment. The crust here is too new for sediments to have had time to accumulate. As the crust is pushed away from the crest, sediments begin accumulating. A cross section shows that younger sediment deposits form layers on top of older deposits. The oldest sediments on the ocean floor are at the greatest distance from the ridges, sitting directly on top of the basaltic-rock crust. These sediments were the first to be deposited.
Scientists also learn a great deal by classifying sediments based on where they originate. As you may imagine, this is important for understanding how sediments are created, the environmental conditions that existed at the time they were created, and what sorts of geologic deposits they will become. This is why the science of determining sediment origins is one of the cornerstones of paleoceanography. Sediments may be classified based on origin into four categories: lithogenous, biogenous, hydrogenous, and cosmogenous.
Lithogenous sediments come from the land (from the Greek lithos, meaning "stone," and Latin generare, meaning "to produce"). They result primarily from erosion by water, wind, and ice. Eroded rock and mineral particles are then carried into the sea. Other lithogenous sediments enter the sea from landslides and volcanic eruptions. Lithogenous sediments make up the majority of sediments found near continents and islands, reflecting their origins on land. It is estimated that rivers carry up to 15 billion metric tons of lithogenous sediments into the ocean annually. Human activities such as agriculture and construction disturb the land and contribute to erosion. Another estimated 100 million metric tons of sediment transfers from land to sea as fine dust and volcanic ash.
Biogenous sediments originate from organisms (from the Greek bios, meaning "life"). The particles in these sediments come from shells and hard skeletons. Although lithogenous sediments represent the largest total volume, biogenous sediments cover a greater area of sea floor. The vast majority of biogenous sediments come from planktonic organisms that obtain siliceous (containing silica) and calcareous (containing calcium carbonate) compounds from seawater. These organisms use the compounds to form shells or skeletons, which later settle to the bottom as sediment when the organisms die. Some sediment comes from large organisms' shells and hard corals. Biogenous sediments are most plentiful where there is a lot of biological productivity and where there are not a lot of other sediments in the water column. Over time, biogenous sediments accumulate into layers. Under the right conditions, organic carbon molecules within these sediments form crude oil (petroleum), and natural gas.
Hydrogenous sediments result from chemical reactions within seawater (from the Greek word hydro, meaning "water"). The reactions cause minerals to precipitate out of solution and form particles that eventually settle on the bottom. The sources of the dissolved minerals vary. Some are produced in the dissolution of submerged rock and sediments. Other dissolved minerals originate from the formation of new crust at mid-ocean ridges, the spewing of chemicals dissolved in hydrothermal vent water, and river runoff. Although mineral precipitation accounts for less than 1% of seafloor sediments, the process produces important mineral deposits, such as ferromanganese and phosphorite nodules.
Cosmogenous sediments come from outer space (from the Greek kosmos, meaning "space"). They are primarily made up of small particles—the size of a grain of sand or smaller—called "cosmic dust." Some of these fragments are thought to result from collisions between objects in space, such as asteroids and comets. Cosmic dust grains, pulled by gravity, continually settle through our atmosphere, similar to how particles settle through water. Cosmogeneous sediments are the least abundant of the sediments. Typically they make up no more than a few parts per million of a marine sediment layer.